ionization interference
/ eye-on-ih-ZAY-shun in-ter-FEER-ence /
Ionization interference is a sneaky way of losing the very atoms you are trying to count. In a hot source, some of your analyte atoms get so excited they lose an electron and become ions; ions absorb and emit at different wavelengths than neutral atoms, so they quietly drop out of the count and your reading comes up low.
More precisely, easily ionised elements — the alkali and alkaline-earth metals especially — partly ionise in flames or plasmas. The hotter the source and the lower the element's ionisation energy, the bigger the loss. Worse, the degree of ionisation shifts with whatever else is in the sample, so the signal becomes unsteady and matrix-dependent.
It matters because it can make results both low and erratic without any obvious sign. The standard fix is an ionisation suppressant (or ionisation buffer): add a large excess of an even more easily ionised element, such as potassium or caesium, which floods the source with electrons and pushes your analyte back to its neutral, countable form.
Measuring calcium in a hot flame, part of the calcium ionises and the signal reads low. Adding a large excess of potassium ions floods the flame with electrons, suppressing calcium ionisation and restoring the full signal.
Atoms lost to ionisation read low; an easily ionised additive buys them back.
In ICP-MS the very ionisation that hurts AAS is wanted, because the instrument counts ions; there the worry flips to elements that ionise too little to detect. Do not confuse ionization interference (atoms turning into ions) with chemical interference (atoms staying locked in compounds).